Polycarbonates, for example from bisphenol A, are mostly amorphous technical thermoplastics with high-quality properties, such as e.g. high transparency, thermal resistance and toughness. The same applies also to aromatic copolycarbonates that are built up, for example, from bisphenol A and a cobisphenol. The production costs of such materials and the level of their properties thus also justify more demanding recycling processes, if old molded parts or production scrap are to be sent for ecologically-necessary and economically-reasonable recyling. The divide between processing costs and economy is considerably more beneficial for aromatic polycarbonates than for many other thermoplastics, so that processes consisting of more than one process step are definitely also worthwhile. However, efforts are always being made to find processes that are simpler and more economic than the known processes, in order to be able to produce more cheaply.
As with other thermoplastics, the level of mechanical and physical properties of polycarbonate depends on the molecular weight. However, production waste, recyclates etc. frequently do not, or no longer, possess the required molecular weights. Direct material recycling of production waste or recyclates is therefore possible only to a very limited extent.
When recycling polycarbonate residues, production wastes, remainders, recyclates and similar polycarbonate compositions, it is therefore desirable and essential to increase the molecular weight to a sufficient level for the projected new use. So, for example, low-molecular production scrap from PC production for Compact Discs could be increased to the molecular weight range required for injection molding. Or the average molecular weight of PC recyclate from the de-lamination of Compact Discs should be increased sufficiently to allow the material to be used, e.g,. as a component in the production of PC/ABS blends.
There is little reference in the literature to condensation of polycarbonate molding compositions destined for chemical-material recycling. Thus EP-A 931 810 discloses a process for increasing the molecular weight of decomposed low-molecular polycondensates such as polyamides, polyesters and polycarbonates using reactive chain lengtheners. These chain lengtheners react in the plastic melt, e.g, in an extruder, under conventional compounding conditions, with the functional chain ends of the polymer. Special bisepoxides are mentioned as chain lengtheners, alone or in combination with epoxides, bisoxazolines, dicyahates, tetracarboxylic acid dianhydrides, bismaleic imides and carbodiimides amongst others. However, no example is given for the function of the process with polycarbonate. The disclosed trials showed no increase in the molecular weight of polycarbonate using the process disclosed in EP-A 931 810.
DE-PS 43 26 906 provides a process for the chemical recycling of polycarbonate by transesterification with hydroxy compounds, in particular phenol, until bisphenol is obtained and the esterified carbonate unit, followed by resynthesis of polycarbonate in the melt. Although this process produces bisphenol and the carbonate unit, the polycarbonate is then completely decomposed by transesterification. In addition to the decomposition of useful bonds, which must then be built up again, the process method is also complex and expensive.
DE-OS 42 40 314 differs from the above patent specification substantially by prior decomposition of the polycarbonate to oligomers, by transesterification with low-boiling monophenols. Then a higher-viscosity oligomer with a particular content of OH terminal groups is produced first from the decomposition product and optionally added diarylcarbonate by recondensation with the splitting off of the monophenol, which, in the final stage, is then polycondensed in the melt under more rigorous reaction conditions to form the desired polycarbonate. Here too the polycarbonate is first decomposed, which necessitates an additional process step. A further disadvantage is that oligomers with OH terminal groups are significantly more susceptible to thermal and oxidative loading than the corresponding polymers and rapidly become discoloured and damaged. The condensation process is therefore sensitive and must be subjected to complex controls to achieve a precise reaction that minimizes such effects.
Finally, DE-OS 44 21 701 discloses a process for the chemical recycling of polycarbonates by decomposition with diaryl carbonates to form oligomers. After they have been crystallised in a particular solvent, cleaned and dried, these are re-condensed to form polycarbonate, optionally with the addition of bisphenols and a catalyst. The disadvantages of this recycling process are the same as those of the processes according to DE-PS 43 26 906 and DE-OS 42 40 314, described above.
On the basis of this prior art, the object was therefore to provide a process by which the molecular weight of polycarbonates may be increased as simply and efficiently as possible.